Just want to leave a message about something NOT to try.
I installed the Microsoft Windows Server* 2012 driver on my Win 7 box [ver 18.104.22.168.3630] [distributed with the server version]
[This is not recommended!]
It seemed to be working fine, but I ran into an H.264 transcode job that got stuck hard in the first frame on frame encoding with MFX_WRN_DEVICE_BUSY.
Installing a regular Win 7/Win8.x version seemed to fix the issue.
Just a warning to other adventurous souls.